Vadim,
Even when you have a really, really fast connection, having more than a few users in a pull-down list gets very frustrating very quickly. It takes a heckuva long time to grab 20,000 users (in my particular case) from the database, to say nothing of the time that it takes to send them via HTTP and have the browser format them into a form. I ran Lynx on a local machine and still had to wait a fair amount of time to get all of the results. (Perhaps this says more about the overloaded PostgreSQL server at hub.org than anything else, but it wasn't fun regardless.)
So yeah, a fast connection is a big help overall -- but it's no substitute for a good UI.
I wouldn't mind a kernel parameter that lets you specify the maximum number of items that appear in a selection list before it turns into a two-stage selection. That will allow individual sites to tune the UI to their needs.
